Originally Posted by
G-BOAC
Regardless of what BD should be doing as a goodwill gesture for downgrading a Gold of all people, you should be entitled to the EU downgrade compensation - 75% of 'ticket price'. Some incredibly tedious reading on the BA forum if you fancy it with your pint:
It's only 75% on flights over 3500km. If the OP's flight was a UK domestic for example, or other route less than 1500km, the refund is only 30%.
Saying that, and it hasn't happened to me on bmi in many years, I still make a point of noting the cheapest economy fare available on flybmi.com at the time I'm booking a C ticket. Last time I asked for the difference in the two
+ tip 
and received it. The tip was the usual £50 bmi voucher.